Process for processing red mud and producing rare-earth metal salts

ABSTRACT

The invention relates to a process for processing red mud, said process comprising the following steps: (a) providing red mud waste of alumina production (in the following: red mud); (b) further, providing waste containing organic material, and converting to synthesis gas by high temperature pyrolysis; (c) converting the sodium oxide present in the red mud in the form of soluble glass to sodium carbonate with carbonic acid; (d1) magnetizing the Fe 2 O 3  hematite ferric oxide present in the red mud and separating the anti-ferromagnetic Fe 2 O 3  hematite ferric oxide from the remained slurry by magnetic separator; (d2) converting the Fe 2 O 3  hematite ferric oxide present in the red mud to Fe 3 O 4  magnetite ferrous ferric oxide with synthesis gas, and separating the Fe 3 O 4  magnetite ferrous ferric oxide from the remained slurry by magnetic separator; (e) treating the remained slurry obtained in steps (d1) or (d2) with strong acid, thus obtaining metal sulphate solution and SiO 2  and TiO 2  suspension.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a process for processing red mud, and producingrare-earth metal salts.

BACKGROUND OF THE INVENTION

The current global aluminum production is based on the Bayer processpatented 1892, in which the alumina content is extracted from thesedimentary bauxite-mineral, named bauxite.

The principle of the method is that the grounded bauxite is digested bycaustic (NaOH) alkaline cooking, and the alumina (aluminum oxide),aluminum oxide hydrates, gallium is recovered from the resulting slurry.

The residual alkaline slurry is called red mud. This slurry is washed inorder to reduce the alkalinity, and to recover the sodium hydroxidecontent, sometimes dried and collected in red mud ponds.

These red mud ponds are hazardous waste sites, because of the corrosiveeffect of red mud due to its residual alkalinity, on the other hand,because the residue of the very fine-grained (clay) bauxite is alsofine, and very dusty. This dusting out is typically very harmful,belonging to the category of PM 10 particles causing silicosis, asthma,lung cancer, and to the category of PM 2.5 particles causingcardiovascular diseases, and accumulating in the body. The airborneparticles are the air-distributed fine-grained dust of less than 10 μm(particulate matter, PM), the classification of which is performed basedon the particle size, where categories PM 10 and PM 2.5, respectively,correspond to particle sizes less than 10 μm, and 2.5 μm.

Furthermore, the metal (hydroxide) and caustic soda residual alkalinitycontent of the red mud slurry, together with the leachate, infiltrateinto the soil and the drinking water/natural water bases, andcontaminates them.

For these reasons, it would be necessary to fully eliminate the red mudponds. However, to date no profitable solution has been found that wouldmake this, and in such a large volume possible, therefore billions oftons of red mud is deposited in the world, occasionally having spilledout, thereby causing industrial disasters.

This is despite the fact that the red mud contains a lot of importantindustrial raw material metals. Nearly half of it is iron oxide, it hashigh content of titanium dioxide, but it contains very importantrare-earth metals, scandium, yttrium and gallium as well. Thecomposition of an average, in terms of its useful material content,rather poor quality red mud waste [after the extraction of gallium(Ga)], based on the common measurements of MAL Hungarian AluminumProduction and Trade Company and MTA/VE Hungarian Academy ofSciences/University of Veszprem is as follows:

The composition of red mud (MAL and MTA/VE):

-   -   Fe₂O₃(iron oxide) 40-45% by weight, this gives the colour of the        red mud; MTA/VE 37% by weight;    -   Al₂O₃(aluminum oxide) 10-15% by weight MTA/VE 14.3% by weight;    -   SiO₂ (silicon dioxide) 10-15% by weight; present as sodium- or        calcium-alumina-silicate, MTA/VE 20% by weight    -   CaO (calcium oxide) 6-10% by weight MTA/VE 7.7% by weight    -   TiO₂ (titanium dioxide) 4-5% by weight MTA/VE 3.8% by weight    -   Na₂O (bound soda) 5-6% by weight MTA/VE 4.8% by weight    -   MgO MTA/VE 0.53% by weight    -   other materials, such as rare-earth metals: 1 ton of red mud        contains 1 to 3 kg of rare-earth metals, and scandium Sc,        yttrium Y, and if it is not obtained during the production of        oxide hydrate, about 0.1 kg/ton of Ga gallium as well.

The amount of rare-earth metals and other metals in red mud are usuallyas follows:

20 to 30 ppm of gallium, 55 ppm of scandium, 120 ppm of yttrium, 240 ppmof lanthanum, 450 ppm of cerium, 10 ppm of praseodymium, 190 ppm ofneodymium, 23 ppm of samarium, 160 ppm of gadolinium, 40 ppm ofmolybdenum, 410 ppm of zinc, 500-550 ppm of chromium, 55 ppm of cobalt,42 ppm of uranium, 50 ppm of thorium, 1950 ppm of manganese.

The problem is that the concentration of each type of useful material(for all types of the components) is low as compared to theconcentration of ores directly mined for the production of this materialtypes. Therefore, the processing of red mud uneconomical.

According to the state of the art, a number of attempts have been madefor the utilization of red mud, and for the recovery of the usefulmetals of red mud.

Typical red mud elimination method was the use of the red mud as soilconditioner material, for example by improving alkaline soils, but thismethod has not spread.

Taking advantage of the clay mineral nature of red mud, experimentationis going on today for the use as the construction material, both in thecement industry, and in the utilization as housing block (D Y Liu etal.: Materials 5, 1232-1246 (2012). H. Gu et al.: Waste Manag Res 30(9), 961-965 (2012); W. Liu et al.: J. Hazardous Materials, 161 (1),474-478 (2009)). There were initial success, but for example, theAustralian laws prohibited the housing-purpose use of the relativelyhigh radioactivity housing blocks. It does not seem to spread out a bitfar-fetched either option, the red mud application for road repair.

In order to utilize the metal materials of the red mud, the researchersmostly concentrated, and recently still focus on obtaining the iron(oxide) (M S Rukhlyadeva et al.: Inorganic Synthesis and IndustrialInorganic Chemistry (Russian J. of Applied Chemistry) 88 (3), 377-381(2015). Jayassankar K. et al.: Int J. Minerals, Metallurgy andMaterials, 19 (8), 679-684 (2012)).

It is a rather hard approach, because after obtaining the iron oxidecontent of the red mud, it should be competitive in the market, both interms of price, and of the metallurgical quality with the iron ores,which fact already questions the economics of the idea, and thus ofviability on industrial scale.

Apart from the market competitiveness with iron ores, successfulattempts have been made (for example, in the Alumina Industry and SteelResearch Institute of the Hungarian Academy of Sciences) for recoveringthe iron (oxide) content of the red mud, such that the red mud slurrywas heated with charcoal, and the Fe₂O₃ hematite ferric oxide content ofthe slurry, by using the resulting carbon monoxide, was reduced to Fe₃O₄magnetite ferrous ferric oxide. Then, the magnetite was taken out fromthe system using a magnetic separator, and after being pelleted, it wasrecycled in the metallurgy (U.S. Pat. No. 9,199,856 B2, X, Li et al.:Trans Nonferrous Met Soc China 25, 3467-3474 (2015)).

Others modified the same procedure such that originally a carbonmonoxide reducing gas was led onto the red mud, still others hydrogenled onto the red mud (W. Liu et al.: J. Hazardous Materials, 161 (1),474-478 (2009) again, F. Kauss et al.: Chemie Ingenieur Technik 87 (11),1535-1542 (2015)).

The Russian Rosintech Res. Inst. solved this problem such that they ledthe synthesis gas from an aqueous breakdown of natural gas (methane) tothe mud (CH₄+H₂O=CO+3H₂) (Chemosphere 78 (9), 1116-1120 (2010)).

CN 101463420 A publication document discloses a method for comprehensiveutilization of red mud. In its first step, the red mud is mixed withcharcoal and calcium chloride, and heated for 3 hours at 1100° C. TheGaCl₃, TiCl₄, and ScCl₃ components are separated by aqueous washing, andthen the residual slurry is separated to magnetic and non-magneticslurry by magnetic separator. The non-magnetic portion is used for theproduction of building materials. To this the rare-earth metals arerecovered after the addition of sodium carbonate or an aqueous solutionof crystalline oxalate by filtration and drying. To obtain the solubleScCl₃ GaCl₃ components, the material is washed, then soaked for 2 hours,filtered, and the scandium is precipitated by the addition of oxalicacid crystals. After filtration, the Ga(OH)₃ and Ti(OH)₄ components areprecipitated with aqueous ammonia. Then, hydrochloric acid is added, andthe material is extracted with P₂O₄ reagent. After re-extraction of theacidic solution, it is heated for 3 hours at 800° C. U.S. Pat. No.6,248,302 B1 patent document discloses the treatment of red mud forrecovering the metal content. According to the method, the iron,aluminum, silicon and titanium components are precipitated by acidicdissolution, heating and washing with water.

In these procedures the expensive reduction gas is an important elementof the costs, but besides this, the red mud reduction gas system shouldalso be heated to a temperature of about 500° C., which requires largeamount of natural gas, which, considering the gas prices as compared tothe iron ore, makes these pellets uncompetitive in price in the market.(Neither the energy need of pelletization, nor the profitability of theprocessability of the thus obtained iron oxide in fluidized bed are evenconsidered.)

Another disadvantage of the known processes is that the recovery of themetal content is difficult, and in particular not complete. Primarily,recovering the iron content in the form of iron oxides is problematic,where the residual iron content makes it difficult to recover the othermetal components.

The preparation of the rare-earth metals is carried out from mineralores, wherein the rare-earth metal generally exist as oxides orsulfates. The ores are enriched with gravity, magnetic and electrostaticseparation and ore preparation methods, because the concentrations arerarely sufficient for the direct chemical digesting. The chemicaldigestion is carried out in aqueous alkali or acid. Recently ionexchange and the liquid-liquid extraction processes have widely spread(E. Bour-bos et al.: 1st European Rare-earth Resources Conference,Milos, Sep. 4-7, 2014).

It is also known that in the daily life wastes containing organicmaterials are in large quantities formed, which, in some cases, can bequite dangerous. The processing of them is generally performed bythermal decomposition carried out at high temperature (pyrolysis), inwhich synthesis gas with H₂+CO composition is produced (EP 2638130 A1,WO 2009/151180 A, US 2011039956 A1).

The Problem to be Solved

There is need to develop a technique that enables both efficientprocessing of large amounts of red mud, thus the elimination of the redmud ponds hazardous to the environment, on the other hand, allows therecovering of metals and rare-earth metals in the red mud.

More particularly, there is need to develop a method, which allows thefull recovery of iron oxides present in the red mud.

The Solution for the Problem

It has been found that recovery of iron oxides present in the red mud,is hampered by the fact that the iron oxide particles present in red mudare coated with soluble glass enclosure. This soluble glass (sodiumsilicate) is formed from the alkali (NaOH) content, and the SiO₂ contentof the red mud, and surrounds the iron oxide particles in the form of agel. The resulting soluble glass enclosure inhibits the separation ofiron oxide, and thus makes it difficult to complete the processing ofthe red mud, and the subsequent recovery of the metal content of it.

We have found that the above problem is advantageously solved, if priorto the removal of the iron oxides, the soluble glass enclosure isdecomposed by the transformation of soluble glass to sodium carbonateand silicic acid.

In addition, the economics of the process can be substantially improved,if the processing of red mud and simultaneously recovering metals andrare-earth metals present in the red mud, is coupled with wastemanagement process of the wastes with organic content.

BRIEF DESCRIPTION OF THE INVENTION

An advantage of the process according to the invention is that it allowsthe complete removal of iron oxides by the breaking down the solubleglass enclosure surrounding the iron oxide particles, and thus theeconomical utilization of the iron content.

A further advantage of the process according to the invention is theconcentrating of the extremely popular and expensive rare-earth metalcontent of the red mud in such a salt concentrate, which is thenmarketable and searched raw material for the rare-earth-metal vendor anduser companies.

A further advantage of the process according to the invention is that ismakes the processing of the red mud economical, so that itsynergistically combines the processing of wastes containing organicmaterials, said materials generated in daily life, and sometimes beingvery dangerous, with the utilization of red mud waste. As a result ofthe process according to the invention, all variables of the red mud inponds can be eliminated by this process, said technology also producingan outstanding high profit simultaneously.

Accordingly, the subject matter of the present invention is a processfor recovering the iron oxide content of red mud, said processcomprising the step of converting the sodium oxide present in the redmud in the form of soluble glass to sodium carbonate with carbonic acid.

Furthermore, the invention relates to the utilization of red mud, saidprocess comprising the the following steps:

(a) providing red mud waste of alumina production (in the following: redmud);

(b) further, providing waste containing organic material, and convertingto synthesis gas by high temperature pyrolysis;

(c) converting the sodium oxide present in the red mud in the form ofsoluble glass to sodium carbonate with carbonic acid;

(d1) magnetizing the Fe₂O₃ hematite ferric oxide present in the red mudand separating the anti-ferromagnetic Fe₂O₃ hematite ferric oxide fromthe remained slurry by magnetic separator;

(d2) converting the Fe₂O₃ hematite ferric oxide present in the red mudto Fe₃O₄ magnetite ferrous ferric oxide with synthesis gas, andseparating the Fe₃O₄ magnetite ferrous ferric oxide from the remainedslurry by magnetic separator;

(e) treating the remained slurry obtained in steps (d1) or (d2) withstrong acid, thus obtaining metal sulphate solution and SiO₂ and TiO₂suspension.

In one embodiment of the process according to the present invention, theprocess further comprises the the following steps:

(d3) treating the Fe₂O₃ hematite ferric oxide obtained in step (d1)and/or the Fe₃O₄ magnetite ferrous ferric oxide obtained in step (d2)with the synthesis gas obtained in step (b), in which pure iron (Fe) isobtained.

In a further embodiment of the process according to the presentinvention, said process further comprises the steps of:

(d4) iron (Fe) obtained in step (d3) is treated with carbon monoxide gas(CO), in which the Fe(CO)₅ iron pentacarbonyl is obtained.

In a further embodiment, the process according to the present inventionfurther comprises the steps of:

(e1) the metals and rare-earth metals are recovered from the metalsulphates and rare-earth sulphates in the metal sulphate solutionobtained in step (e) in a conventional manner.

In a further embodiment of the process according to the presentinvention, said process further comprises the steps of:

(e2) the SiO₂ and TiO₂ slurry obtained in step (e) is separated byconventional techniques, obtaining pure SiO₂ quartz sand, and pure TiO₂titanium dioxide.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the various steps of the process and the material flowsaccording to the invention as a block diagram.

FIG. 2 shows the material balance, which repeats the above indicatingthe specific volumes on the basis of 1000 tons of red mud input.

FIG. 3 shows a schematic arrangement of an apparatus for performing theprocess according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, this invention relates to a process for recoveringthe iron oxide content of red mud, comprising the step of converting thesodium oxide the red mud in the form of soluble glass (Na₂SiO₃), tosodium carbonate with carbonic acid.

The red mud used in the process according to the present invention isthe alkaline slurry resulting from the slurry produced by the alkalinecooking of bauxite, which remains after obtaining the alumina.Preferably the thick red mud collected and deposited in the ponds may beused.

The soluble glass (sodium silicate) is the reaction product of the NaOHsodium hydroxide remaining from the Bayer process and SiO₂ silicondioxide (sand), which usually occurs in the forms of orthosilicate(Na₄SiO₄), metasilicate (Na₂SiO₃), polysilicate ((Na₂SiO₃).) andpyrosilicate (Na₆Si₂O₇). The soluble glass well dissolves in water andalkali, and forms a gelatinous, gel-like material. This gel adheres tothe iron oxide particles of the red mud, and highly impairs theirgas-accessibility. Moreover, it agglutinates the iron oxide particleswith other materials, for which the CaO calcium oxide, MgO magnesiumoxide, and the like may be mentioned as examples. To obtain the ironoxide, this soluble glass enclosure should be broken down first. Thesoluble glass immediately decomposes in acidic media. To achieve this,any inorganic or organic acid such as sulfuric acid, phosphoric acid,carbonic acid and the like can be used. Due to its easy applicability,and in order to avoid the introduction of any foreign material into thesystem, breaking down of the soluble glass coverage surrounding the ironoxide particles present in the red mud, may preferably be achieved byusing carbonic acid.

In the course of the carbonic acid treatment, the soluble glasssurrounding the iron oxide particles glass is converted with H₂CO₃carbonic acid to Na₂CO₃ sodium carbonate and H₂SiO₃ metasilicic acid(which is easily decomposed into water and SiO₂ silica sand). Thereaction of sodium silicate and carbonic acid is shown in the followingreaction scheme.

Na₂SiO₃+H₂CO₃=Na₂CO₃+H₂SiO₃

The resulting products are no longer bound to the iron oxide particles,and particles of other materials are not adhered thereto, but are rathera separate part of the remaining slurry.

For the conversion the red mud is heated in a drum-type furnace usuallyat 200-400° C., and typically at about 300° C. with the addition of CO₂carbon dioxide, wherein the added CO₂ carbon dioxide, as a result themoisture content of the red mud, converts to carbonic acid and effectsthe breaking down of the soluble glass coating.

Before the carbonic acid treatment, the red mud deposited in the pondsis prepared. In doing so, it is excavated from the ponds usingexcavators, or other appropriate means, and with a closed-off conveyorbelt supplied with dust-free technology, it is forwarded to a productionschedules warehouse. Here it is stored out of reach of rain and wind forapproximately one week. From this location it is fed into the drum-typefurnace in such a manner that the added red mud is screened (in order toweed out any foreign bodies), then it is pre-powdered, in order tofacilitate the subsequent operations for the gas permeability byshredding the eventually adhered blocks, lumps off. (In case of an about200000 t/year production, about 90-150 t/h feedable red mud should beprestored, if we take into account the operational limits of theoutlined procedure.) The prepared red mud is fed in the drum-typefurnace. The rotating drum-type furnace filled with the prepared red mudis then closed, and at room temperature, while rotation it is washedwith an inert gas (preferably nitrogen). The displacement of air ischecked using an oxygen analyzer connected to the gas outlet. After thedisplacement of oxygen (air) (the loss of outflow at the outlet), thenitrogen is turned off, and the slurry in the drum-type furnace, whilerotating the drum-type furnace, is run at a temperature of 100-150° C.in order to remove the remaining water content from the ponds, and toconvert the red mud slurry to a dry powder for the additionaloperations. The drying is controlled by measuring the amount of steam bya device equipped to the gas outlet tube of the furnace, while thetemperature is shown by a built-in thermometer and it is checked by thecontrolled heating of the furnace.

The amount of carbon dioxide to be fed in the drum-type furnace can beestimated on the basis of the SiO₂ and Na₂O content determined by arepresentative chemical analysis prior to the processing. In practice,the feeding of carbon dioxide in the drum-type furnace is continueduntil the outflow of water vapor content through the gas compositionanalyzer of the drum-type furnace is ceased.

Then, the powder-slurry of the red mud so modified is cooled to roomtemperature, preferably making use of the cooling effect of the carbondioxide and/or nitrogen flushing.

For the task use of a rotating drum-type furnace is suitable, which isheated by 380V 50 Hz industrial electricity, whereas the heating power(the current) is controlled by setting of the desired temperature.

After breaking down the soluble glass coverage surrounding the ironoxide particles present in the red mud, the iron oxide content may bepractically fully recovered.

The present invention further relates to a process for the utilizationof red mud, said process comprising the above-specified steps (a) to(e).

The red mud used in step (a) according to the invention is the residualalkaline slurry resulting from the slurry produced by cooking of thebauxite after the extraction of alumina. As preferred example, the thickred mud collected and deposited in the ponds a may be used.

In step (b) according to the present invention the waste containingorganic material used in the production of synthesis gas can be forexample waste paper, rice husking residues, straw, hemp, flax, theproducts of there, and the like. The waste containing cellulose is dryor wet waste, especially waste of about 10% by weight moisture content.

The waste of the organic content can also be waste containing dioxin andfuran, such as waste oil, transformer oil, agricultural chemicalsresidue, and the like, or other industrial hazardous waste, such asbated leather waste, oil sludge, contaminated gas black, tarry waste,plastic and the like.

The composition of the synthesis gas used in the invention is CO+H₂, andis generally in 1:1 mole ratio CO+H₂. The production of synthesis gas iscarried out in a conventional manner. In doing so, it is preferred thatthe waste containing organic matter is heated in furnace, forge orplasma furnace at temperatures above 1000° C. isolated from air, and theresulting synthesis gas is purified in the usual manner and thentransferred from tank or for direct use.

In one embodiment according to the present invention, the pyrolysis ofthe organic material containing waste is performed in a plasma energypyrolysis system (e.g. PEPS Plasma Energy Pyrolysis System), brieflyplasma forge.

The feature of plasma forge is that it is the most suitable for thedisposal of hazardous waste (BAT technology). Accordingly, in this casethe synthesis gas can be produced from wastes for example containingdioxins and furans (waste oils, transformer oils, agricultural chemicalsresidues, and the like), or other industrial hazardous wastes (batedleather waste, oil sludges, contaminated gas black, tarry waste,plastics and the like).

If the resulting synthesis gas differs from the 1:1 CO+H₂ composition,the preferred gas composition is adjusted by addition of the missingspecific gas. For example, in case of excess carbon monoxide, hydrogenis added from a bottle, a tank or by direct water decomposition, untilthe 1:1 molar ratio is reached, while in case of excess hydrogen, carbonmonoxide is added from a bottle, a tank until the 1:1 molar ratio isreached.

In step (c) of the process according to the present invention the sodiumoxide present in the red mud in the form of soluble glass is convertedto sodium carbonate using carbonic acid.

Step (c) of the process according to the invention is preferably carriedout as described above.

Step (d1) of the process according to the present invention consists oftwo parts. In the first part the Fe₂O₃ hematite ferric oxide present inthe red mud is magnetized, and in the second part the antiferromagneticFe₂O₃ hematite ferric oxide is separated from the residual slurry usinga magnetic separator.

The magnetization is carried out in a conventional manner. For example,the preparation of red mud may be passed through two magnetic separatorsconnected in series, wherein in the first magnetic separator the strongmagnetic field will magnetize the Fe₂O₃ hematite ferric oxidecrystallites, and then, the Fe₂O₃ hematite ferric oxide crystallitesexhibiting in this manner (anti)ferromagnetic properties are separatedin the second magnetic separator from the red mud. As a result of this,pure Fe₂O₃ hematite ferric oxide and residual powder mud is obtained.

Alternatively, the red mud prepared is magnetized by heating in aninduction furnace magnetizing. Induction furnaces also induce analternating frequency magnetic field, which is suitable for magnetizingthe paramagnetic hematite in the iron oxide Fe₂O₃ to(anti-)ferromagnetic state.

Preferred is the use of the induction-heated drum-type furnace, becauseit next to changing the structure of the red mud, pre-magnetizes theoriginally paramagnetic Fe₂O₃ hematite particles by its induced magneticfield, and this is an important step to bring the hematite particles to(anti)ferromagnetic state.

The pre-magnetized hematite particles may also be magnetized in amagnetic field formed by standard 380V 50 Hz, but ideal is using aroundnearly 1 kHz alternating current and a magnetic separator having about 1Tesla magnetic field power.

After the magnetization the powder mud can be led to a magneticseparator, which is also preferably a device operating according to theprinciple of magnetic induction, established for the separation offerromagnetic and (anti-)ferromagnetic materials, or in the same devicedesign as that of the device with “magnetizing” function. They aresufficiently high-productivity machines to be able to separate the redmud powder appearing on the output of the drum-type furnaces.

In this magnetization process, wherein the hematite→(anti-)ferromagnetichematite conversion is carried out, the iron oxide content is presenttypically in the form of Fe₂O₃ hematite ferric oxide, but in smalleramounts Fe₃O₄ in the form of magnetite ferrous ferric oxide can also bepresent. This does not affect adversely the goodness, efficiency of themagnetic separation process.

In another embodiment of the process according to the present inventionthe Fe₂O₃ hematite ferric oxide crystallites are first pre-magnetized.This can be accomplished for example, by such a manner that thedrum-type furnace applied for converting the sodium oxide present in theform of soluble glass in the red mud to sodium carbonate is adjusted toinduction heating, wherein the induction field will pre-magnetize theFe₂O₃ hematite ferric oxide crystallites. The magnetization of thepre-magnetized crystallites is then carried out e.g. as described above.

Step (d2) of the process according to the invention also consists of twoparts. In the first part the Fe₂O₃ hematite ferric oxide present in thered mud is converted to Fe₃O₄ magnetite ferrous ferric oxide, which canbe added to magnetic separator, and in the second part the magneticFe₃O₄ ferrous ferric oxide is separated from the remaining slurry bymagnetic separator.

The red mud is fed into a specially designed rotary drum-type furnace.Said drum-type furnace should be able to ensure the explosion-free useof synthesis gas.

The rotating drum-type furnace filled with red mud is closed and flushedwith an inert gas (preferably nitrogen) at room temperature withrotation. The displacement of air is checked by an oxygen analyzerconnected to the gas outlet. After the displacement of oxygen (air) (theloss of outflow at the outlet) the nitrogen is turned off, and theslurry in the drum-type furnace, while rotating the drum-type furnace isrun at a temperature of 100-150° C., to remove the remaining watercontent from the ponds, and a dry powder of the red mud slurry befurthered for the next operations. The latter two operations can also becombined. The drying is controlled by measuring the steam content usinga device equipped to the gas outlet tube of the furnace, while thetemperature is shown by a built-in thermometer, and it is checked by thecontrolled heating of the furnace.

After sufficient drying, the temperature of the rotating drum-typefurnace is raised to a temperature of 450-550° C. necessary for thereduction to magnetite with synthesis gas (because the reactiontemperature of the reduction is about 500° C., however, the hematiteloses its antiferromagnetism above 600° C.).

Then, the 1:1 mole ratio synthesis gas CO+H₂ is allowed onto the red mudpowder system of the rotating and heated drum-type furnace. Thesynthesis gas is fed in a predefined volume in the system to prevent theover-reduction of iron oxides to FeO/Fe₂O₃ in 1:1 mole ratio (becausethe relationship between magnetite and hematite is: Fe₃O₄→FeO×Fe₂O₃).

The amount to be added is determined based on the prior andrepresentative analytics of the hematite content in the red mud to beprocessed, and weight ratios of the 2Fe₂O₃+(CO+H₂)→4FeO+H₂CO₃ chemicalreaction equation. (See our attached block scheme showing also the massbalance).

This reduction heating in the synthesis gas is continued monitoring ofthe hydrogen analyzer equipped to the gas exit, until the hydrogen isconsumed. Then the red mud is cooled back to room temperature (ambienttemperature), preferably with rotation during nitrogen purge.

The total processing time of the drum-type furnace operation is about 2to 3 hours (therefore the calculated production capacity of thedrum-type furnace operation for a 200000 t/y red mud production lines isaround 90-150 t/h).

For the above process an induction-heated rotating drum-type furnace ispreferably used.

On one hand, such heaters and the temperatures achieved with them arewell controllable on the other hand, the induction furnaces also inducean alternating frequency magnetic field, which is suitable for themagnetization of the paramagnetic Fe₂O₃ hematite ferric oxide toanti-ferromagnetic.

Therefore, if the chemical transformation as described above (thepreparation of magnetite from the hematite) is not perfect, andremaining Fe₂O₃ would be in the system, it is preferably magnetized bythe magnetic field, and it allows the separation by the suitablemagnetic separator with induced magnetic field (together with themagnetite). By the repeated addition of the powder already having passedthrough the separator and having magnetically not selected to theseparator, the magnetized hematite remains may completely separated fromthe slurry.

For the implementation of the the task a rotating drum-type furnace issuitable, which is heated by 380V 50 Hz industrial electricity, whereasthe heating power (the current) is controlled by setting of the desiredtemperature.

In one embodiment of the invention, the red mud is treated with thesynthesis gas produced in step (b) in which the sodium oxide in red mudin the form of soluble glass is is converted to sodium carbonate [step(c)], and simultaneously the Fe₂O₃ hematite ferric oxide is converted toFe₃O₄ magnetite ferrous ferric oxide, which can then be fed to magneticreduction separator [step (d2)]. One principle of the process accordingto the the invention is that at least the reduction gas, and optionallythe gas providing for the energy need of heating up to the reactiontemperature is covered by the production of synthesis gas derived fromwaste.

The separation of Fe₂O₃ hematite ferric oxide by the magnetization andthen by magnetic separator implemented according to the step (d1) is notalways sufficient for the complete removal of Fe₂O₃ hematite ferricoxide present in the red mud. In order to remove the residual Fe₂O₃hematite ferric oxide the red mud is is treated according to step (d2)with the synthesis gas obtained in step (b).

In order to implement the treatment with synthesis gas, synthesis gas ispassed into the red mud, and it is reduced at elevated temperatures. Thereduction is preferably carried out in the manner described above.

The magnetized Fe₂O₃ hematite ferric oxide obtained in step (d1) and/orthe Fe₃O₄ magnetite ferrous ferric oxide obtained in step (d2) isseparated from remained slurry, in which pure Fe₂O₃ hematite ferricoxide or Fe₃O₄ magnetite ferrous ferric oxide is obtained. Theseparation is preferably performed, for example in a magnetic separator.

In one embodiment of the process of the present invention the resultingFe₂O₃ hematite ferric oxide, or Fe₃O₄ magnetite ferrous ferric oxide istreated in the synthesis gas produced in step (b), during which pure,high surface activity, chemically highly active iron Fe is obtained[Step (d3)]. To this drum-type furnace is preferably used, which isheated to 800 to 1000° C. under a stream of synthesis gas.

In a further embodiment of the process according to the presentinvention, the Fe obtained in the form of high surface activity powderis treated with carbon monoxide gas, (CO), in which the Fe(CO)₅ ironpentacarbonyl [step (d4)] is obtained.

The method comprises reacting iron Fe with CO carbon monoxide usually at180 to 200° C., and at a pressure of 100 to 200 atm. In the processaccording to the present invention this can be achieved at a temperatureof 110 to 140° C. and at a pressure of 10 to 25 atm, since iron powderreduced with hydrogen is used in place of the usual sponge iron, saidpowder iron exhibiting unique surface and hence chemical activity. Indoing so, as CO source the CO component of the synthesis gas can beused.

The residual slurry obtained in step (d1) or (d2) of the processaccording to the present invention, is treated with a strong acid suchas hydrochloric acid or sulfuric acid, in particular with 96% industrystrength aqueous sulfuric acid [step (e)]. In doing so, the metal oxidesand rare-earth metal oxides present in the residual slurry are convertedto metal sulphates and rare-earth metal sulphates, and the metals andrare-earth metals are recovered from the resulting metal sulphates andrare-earth metal sulfates in a conventional manner [step (e1)].

In one embodiment of the process according to the present invention, theremaining slurry used for the acid treatment is first diluted with waterin the ratio by weight from 1 to 2, preferably 1.2 to 1.8, then it isstirred at 20 to 100° C., preferably 30 to 80° C. for 0.5 for 3 hours,preferably for 1-2 hours. At this point the metal oxides form analkaline suspension, in which neither of titanium dioxide, nor silicondioxide is dissolved. To the thus treated slurry, portions of a strongacid, particularly 96% industrial strength sulfuric acid are added understirring, during which the mixture is brought to a neutral state aroundof about pH=7. The neutralized slurry is aged, in which the rare-earthmetal sulphates remain in solution, and the titanium dioxide, andsilicon oxide settles.

The solution containing the rare-earth metal sulphates are separated bye.g. decantation, suction or centrifugation, and the solution is whollyor partly evaporated at a temperature up to 100° C. Thus, a concentratedsalt solution or dry metal salts are obtained, which may be separated toindividual metal salts in a conventional manner.

The SiO₂ and TiO₂ suspension obtained in step (e) of the processaccording to the present invention is separated by conventionaltechniques, obtaining pure SiO₂ quartz sand and pure TiO₂ titaniumdioxide [Step (e2)]. This is preferably achieved according to EP A1 HUP1200075 patent publication document.

The iron oxide produced according to the process of the presentinvention may be used in any suitable manner. Examples of the fields ofapplication are as follows: a catalyst, a pigment, a water treatmentflocculent with dissolution with hydrochloric or sulfuric acid, ormetallurgical recovery.

Likewise, pure iron prepared by the process according to the presentinvention, iron pentacarbonyl, and rare-earth oxides may be used in anysuitable manner.

Similarly, the titanium dioxide may be used in any suitable manner.

The quartz sand can be used as washed sand for building construction,may be a starting material for refractory lining bodies, glassproduction component, high-purity casting sand, but it is suitable forceramic purposes and for pressing building blocks as well.

FIG. 3 shows schematically the main units of a 100 apparatus forcarrying out the process according to the invention, and theirarrangement.

The 100 apparatus comprises a 110 drum-type furnace, a 120 CO₂ tank, a130 magnetic separator and a 140 acid-treatment tank, a 145 acid tank, a150 synthesis gas tank, a 160 pyrolysis unit 170 iron settling unit a180 centrifuge and a 190 magnetizing unit.

The 110 drum-type furnace has a 111 red mud inlet (not shown) receivingraw red mud from production unit or red mud ponds, a 112 CO₂ inletreceiving CO₂ from the 120 CO₂ tank through the L4 line, a 114 synthesisgas inlet receiving CO+H₂ synthesis gas from the 150, a synthesis gastank through the lockable L1 pipeline, furthermore a 113 treated sludgeoutlet providing for sludge treated thermally or by induction heating(pre-magnetization). The 110 drum-type furnace may preferably beinductively heated, allowing the magnetization of Fe₂O₃ hematite ferricoxide present in the red mud.

The 130 magnetic separator receives the treated red mud at the 131treated sludge inlet said treated red mud being transmitted from the 110drum-type furnace through the L3 line and the hematite or magnetiteobtained as a result of magnetic separation is discarded at a 132hematite/magnetite outlet, and the residual slurry remaining from thered mud is discarded through a 132 slurry outlet.

If no reduction is performed in the 110 drum-type furnace, theheat-treated slurry is transferred through a lockable L10 pipeline tothe 190 magnetizing unit, which receives the red mud pre-magnetized bythe 110 drum-type furnace through a 191 magnetizing inlet, and themagnetized iron-oxide-containing slurry is transferred through a 192magnetizing outlet terminal, in one also lockable L11 pipeline to the130 magnetic separator, to its second 131′ treated mud inlet. In orderto ensure that the stream of material can be fed to the 130 magneticseparator through the 190 magnetizing unit, the section of the L3pipeline after its branching is also designed for being suitable to beturned off.

The 190 magnetizing unit may be a second magnetic separator, but alsomay be formed as an integrated part of the 130 magnetic separator. Inthe latter case obviously there is no need for the L10, L11 lines.

The slurry formed in 130 magnetic separator is transferred to to the 141slurry-input of the 140 acid-treatment tank through pipeline L5. The 140acid-treatment tank receives the sulfuric acid needed for the acidtreatment at a 142 acid-inlet from the 145 acid tank through thepipeline L7. The metal sulphate salts resulting from the acid treatmentare discarded through the 143 metal sulphate-output, while the remainingslurry is discarded through a 144 residual slurry outlet from the 140acid treatment tank.

The 150 synthesis gas tank stores the CO+H₂ synthesis gas produced inthe 160 pyrolysis unit by the high temperature gasification (pyrolysis)of the waste with organic matter content. The synthesis gas istransferred from the 160 pyrolysis unit to the 150 synthesis gas tankthrough pipeline L9.

The 100 apparatus also comprises a 170 iron depositing unit, whichreceives hematite or magnetite iron oxide from the 130 magneticseparator at a 171 hematite/magnetite inlet, said hematite or magnetiteiron oxide being transferred through the lockable L6 pipeline,furthermore, it receives the synthesis gas needed for the reduction atthe 172 synthesis gas inlet, said synthesis gas is transferred from the150 synthesis gas tank, through the L2 pipeline. The pure iron powderobtained as a result of the reduction may be discarded through the 170iron depositing unit through the 173 Fe outlet.

The 100 apparatus according to the invention also comprises a 180centrifuge, which separates the titanium oxide and sand discarded fromthe 140 acid-treatment tank from the slurry discharged through lockablepipeline L8. Accordingly, the 180 centrifuge has a 181 residual slurryinlet and outlets 182 and 183 for the TiO₂— and SiO₂-outputs.

It should be noted in connection with the pipelines in the 100 apparatusaccording to the present invention, said pipelines connecting therespective components to each other, that is it obvious for the personsof ordinary skill in the art that the respective units how, and withwhat kind of pipelines must be connected to each other in order toensure the appropriate material flow and the required operatingparameters (such as pressure, flow rate, sectioning, and the like).

It is further noted that by closing one or more amongst the lockablepipelines (especially L1, L2, L3, L6, L8, L10, L11 pipelines) a givenunit or even more selected units of the apparatus are detachable fromthe apparatus, thereby enabling the formation of a variety of operatingmodes corresponding to the above-described process, or configurationcorresponding to various embodiments of the invention. The simpleststructure, i.e. the configuration of minimum according to the equipmentis surrounded by dotted lines in FIG. 3.

Finally, it is noted that the processing units in the apparatusaccording to the invention are well known and widely used in thechemical industry, the functional design, material selection, sizing ofsaid units therefore belong to the routine works of the person withordinary skill in the art, and therefore they will not be discussed indetail in the description of the present specification.

The invention is further illustrated by the following examples withoutlimiting the scope of claims to these examples.

EXAMPLES Example 1 Processing of Red Sludge from Almásffizitö Recoveringthe Hematite Content

The composition of red mud deposited in the ponds at Almásfiuzitö (VEABMonography):

Al₂O₃ 15-19% Fe₂O₃ 30-40% SiO₂ 10-15% TiO₂  3-6% Na₂O  6-14% MgO  0.3-1%CaO  3-9% V₂O 0.2-0.4%  P₂O₅ 0.5-1.0%  CO₂  2-3% SO₃  0.8-2% F 0.1-0.4% C 0.15-0.2%  loss due to glowing 15-18%

Trace Elements:

Ga 20-30 ppm Se 55 ppm Y 120 ppm La 240 ppm Ce 450 ppm Pr 10 ppm Nd 190ppm Sm 23 ppm Gd 160 ppm Mo 4 ppm Zn 410 ppm Cr 500-550 ppm U 42 ppm Th50 ppm

In the example, the calculation is made with the average value, and theprocessing of the red mud is shown for 1000 kg (1 t) of startingmaterial.

The red mud with about 10-30% by weight moisture content is added to aheated, rotating drum-type furnace wherein operation is done atatmospheric pressure of 1 bar. The drum-type furnace is inductivelyheated, and 100 kg of CO₂ carbon dioxide is fed therein. By this mannerthe soluble glass is converted to 171 kg Na₂CO₃ sodium carbonate andH₂SiO₃ meta-silicic acid, while also carrying out the pre-magnetizationof the Fe₂O₃ hematite crystallite particles. The red mud slurry madedigestable, and pre-magnetized in induction furnace, dried to powder isrun through the strong magnetic field of a magnetic separator. Thehematite particles thus made ferromagnetic are magnetically separatedfrom the slurry and residue by a magnetic separator. Thus, 350 kg ofFe₂O₃ hematite ferric oxide and 721 kg of residue slurry is obtained.

By treating the resulting Fe₂O₃ hematite ferric oxide powder with 99 kgof CO+H₂ synthesis gas 244 kg of iron metal powder is obtained.

To the 721 kg residual slurry water is added, and it is treated with1625 kg of 96% concentration industrial strength sulphuric acid inacid-resistant and alkali-resistant tanks, under normal pressure, withoccasional stirring at a temperature of 20-100° C. for several hours.The material is chemically neutralized, and the liquid phase containingthe metal salts dissolved, is separated by centrifugation andconcentrated. Thus, 1588 kg of metal sulfate salt, within this about 3.5kg of rare-earth metal sulphate salt is obtained in the form ofsolution, which is, as desired, concentrated by evaporation.

From the resulting suspension after removing the metal sulphate salts bycentrifugation, 125 kg of silica sand SiO₂ and 45 kg of TiO₂ titaniumdioxide is obtained.

Example 2

Processing of red sludge from Almásfuizitö by reducing the hematitecontent 1000 kg (1 t) quantity of the red mud with the composition asdescribed in Example 1 is processed. To this, the hazardous wastecontaining organic matter (waste oil, sludge, leather waste and thelike) collected in this environment is gasified in a mobile PEPS type(Plasma Energy Pyrolysis Systems) hazardous waste disposal apparatus,and the reduction gas and energy need of processing is thus covered.

The Fe₂O₃ hematite ferric oxide content of the red mud is reduced toFe₃O₄ magnetite ferrous ferric oxide. To achieve this, the red mud isfed in a rotating drum-type furnace, and using 72 kg of synthesis gascomposed of 1:1 mole ratio CO+H₂, prepared from wastes it is heated toabout 500° C. At this temperature another 11 kg of CO+H₂ synthesis gasis fed on the red mud for the reduction. Thus, 338 kg of Fe₃O₄ magnetiteferrous ferric oxide and 23 kg of carbonic acid gas are obtained.

To supplement the carbonic acid formed by the reduction, 55 kg of carbondioxide is added to the drum-type furnace, by which, together withcarbonic acid gas formed during the reduction, the soluble glass contentof the red mud is converted to 171 kg of Na₂CO₃ sodium carbonate andH₂SiO₃ metasilicic acid.

The resultant Fe₃O₄ magnetite ferrous ferric oxide is separated from theremaining slurry using a magnetic separator. Thus, besides 338 kg ofpure Fe₃O₄ magnetite ferrous ferric oxide, 721 kg of residual slurry isobtained.

By treating the resulting pure Fe₃O₄ magnetite ferrous ferric oxide with60 kg of CO+H₂ synthesis gas, 244 kg of iron metal powder is obtained.

The resulting 721 kg of residual slurry is treated in acid-resistant,alkali-resistant tubes with 1625 kg of 96% industrial strength sulfuricacid, under atmospheric pressure, with occasional stirring at 200 to400° C. for several hours. Once dissolved, the slurry is chemicallyneutralized with sodium hydroxide, and the liquid phase containing themetal salts dissolved therein, is separated by centrifugation andconcentrated. Thus, 1588 kg of metal sulphate salt, within this about3.5 kg of rare-earth metal sulphate salt in particular is obtained.

After removing the metal sulphate salts from resulting suspension bycentrifugation, 125 kg of silica sand and 45 kg of SiO₂ TiO₂ titaniumdioxide is obtained.

Example 3 Processing of the Red Mud Form the Vietnamese Tanren AluminumFactory by the Reduction of the Hematite Content

The composition of the red mud from the Vietnamese Aluminum Factory isas follows:

Fe₂O₃ 30.8% MnO 0.02% TiO₂ 2.58% CaO 3.51% K₂O 0.11% P₂O₅ 0.22% SiO₂31.7% Al₂O₃ 15.6% MgO 0.27% Na₂O 3.14%

Rare-earth metals (especially Ce, La, and Nd) in total 28.59 ppm, and0.81 ppm of Sc, and 0.99 ppm of Y.

The processing of the red mud is shown for 1000 kg (1 t) of startingmaterial.

To this as waste containing organic material 160 kg of cellulose (about10% moisture containing rice straw and rice husking residue) is gasifiedin drum-type furnace over 1.5 hours at 1000° C., and thus the reductiongas and energy source need of the processing is covered.

The Fe₂O₃ type hematite ferric oxide content of the red mud is reducedto Fe₃O₄ magnetite ferrous ferric oxide. To achieve this, the red mud isfed into a rotating drum-type furnace, and heated to 500° C. with CO+H₂synthesis gas produced from 72 kg of cellulose waste. At thistemperature it is treated with an additional 10 kg of 1:1 mole ratioCO+H₂ synthesis gas. Thus, 298 kg of Fe₃O₄ magnetite ferrous ferricoxide and 15 kg of carbonic acid gas is obtained.

Another 12 kg of CO₂ carbon dioxide is fed into the reduction block. Asa result of both the reduction of magnetite hematite content of the redmud, and the carbonic acid formed from the introduced carbon dioxide,the soluble glass content of the red mud is converted Na₂CO₃ sodiumcarbonate and H₂SiO₃ metasilicic acid.

The resultant Fe₃O₄ magnetite ferrous ferric oxide is separated from theremaining slurry by a magnetic separator. Thus, 723.4 kg of residualslurry obtained next to the 298 kg of pure magnetite Fe₃O₄ ferrousferric oxide.

By treating the resulting pure Fe₃O₄ magnetite ferrous ferric oxide iswith 58 kg of CO+H₂ synthesis gas, 162 kg of iron metal powder wasobtained.

The 723.4 kg of obtained residual slurry is treated in Door tubes with960 kg of 96% industrial strength sulfuric acid, under atmosphericpressure, with occasional stirring at 200 to 400° C. for several hours.Once dissolved, the slurry was chemically neutralized with sodiumhydroxide and the liquid phase containing the metal salts dissolvedtherein are separated by centrifugation and concentrated. Thus, 922 kgof metal sulphate salt, including about 62.7 g of rare-earth metalsulphate salt is yielded.

After removing the metal sulphate salts from the resulting suspension bycentrifugation, 317 kg of SiO₂ quartz sand and 25.8 kg of TiO₂ titaniumdioxide is obtained.

Example 4

The energy consumption of the process according to the invention isdemonstrated for 1 ton of red mud, said energy may be shared into twoparts, one part for heating to the reaction temperature, another part toproduce the reducing:

a.)

Taking a look at the main components of the red mud, it can be seen that

the specific heat of iron oxide is 0.7 kJ/kgK

the specific heat of alumina is 0.7 to 1.1 kJ/kgK

the specific heat of silica sand is 0.8 kJ/kgK

the specific heat of titanium dioxide is 0.7 kJ/kgK;

and there is substantial water content, wherein the specific heat ofwater is 4.2 kJ/kgK, and the specific heat of the steam is 2 kJ/kgK.

Others have measured the specific heat of red mud to 1.31 kJ/kgK.

Considering these facts the specific heat of the red mud slurry isaccepted to approach the 1.5 kJ/kgK value.

When heated to 500° C., the slurry (now taking the energy need of theheating of the drum-type furnace or other technical means andtechnological losses as zero) and based on the Q=c.m.dT equation for 1ton of red mud this value is 672 MJ (where dT=480° C.).

Taking the heating value of synthesis gas 10 MJ/kg, this also needs 67.2kg of synthesis gas.

b.)

At the same time one ton of red mud, according to the tests of MTA/VE,contains 370 kg of hematite ferric oxide. In order to reduce this amountof hematite to Fe₃O₄ magnetite ferrous ferric oxide, we need 17.4 kg of(CO+H₂) synthesis gas. Of course, in practice, we work substantialexcess of gas, and therefore a+b) need is estimated by 85 kg ofsynthesis gas, that is, for the reduction of the hematite ferric oxidecontent of one ton of red mud to magnetite ferrous ferric oxide, 85 kgof synthesis gas composed of (CO+H₂) is needed.

If this amount of CO+H₂ synthesis gas is produced from a cellulose wastehaving about 10% moisture content (paper waste, rice husking residues,straw, hemp, flax, their products, and the like), this is possible withsmaller amounts with drum-type furnaces as well. Then for 1 ton of fedcellulose, approximately 1 ton of produced synthesis gas will beobtained. Since the energy need of the conversion from 1 ton ofcellulose to CO+H₂ synthesis gas is 430.5 kWh/ton, we see that theenergy need of the conversion of 1 ton of red mud hematite content tomagnetite Fe₃O₄ ferrous ferric oxide is about 36.5 kWh, where 304 kgmagnetite ferrous ferric oxide is obtained from 1 ton of red mud.

It is a further advantage of the nearly 1:1 molar ratio CO+H₂ synthesisgas made from e.g. cellulose, is that after the reduction, CO₂ carbondioxide and H₂O water is formed from the oxygen of Fe₂O₃ iron oxide,which is converted to CO₂+H₂O═H₂CO₃ carbonic acid. If the the sodiumsilicate (soluble glass) [orthosilicate (Na₄SiO₄), metasilicate(Na₂SiO₃), polysilicate ((Na₂SiO₃)_(n)) and pyrosilicate (Na₆Si₂O₇)]surrounding the iron oxide particles of the red mud is heated in anacidic medium, then it leaves the system in the form of silica. Thus, ina single operation the sodium silicate surrounding the iron oxideparticles the red mud can be decomposed into sodium carbonate andsilica, where the latter by the exit of water condenses to SiO₂ silicondioxide again.

Both the sodium carbonate and the silica is intimately mixed with theresidual slurry, and the factor hindering the digestion of the hematiteferric oxide particles will cease to exist.

Considering an average bauxite production volume, that is a logisticallymeaningful red mud processing, then we need to calculate with about 200thousand tons of red mud/year processing. Calculating 300 working daysannually, it is 667 tons per day that is the daily energy need is 56.7tons of synthesis gas, or 24.4 MWh.

In two shifts with a 1-ton drum-type furnace is about 6.4 tons per daysynthesis gas can be produced, which represents about 9 pieces ofdrum-type furnaces in the system.

The efficiency can be further increased, if the energy required tooperate the plasma forge is achieved by renewable energy production runin island mode (see HU No. P150070 A1 patent publication document).

The 358 kg of high-purity iron oxide produced per ton of red mud may beused for the production of a catalyst, iron oxide pigments, byhydrochloric or sulfuric acid dissolving a water purifying flocculant,but also can be utilized metallurgically.

However, it is appropriate to market this item at a higher price, higherstages of preparation, and the production of a query an expensivematerial, the Fe(CO)₅ iron pentacarbonyl is advisable.

It is noted that the reduction of hematite content of the red mud isdirected in such a way that the 48 kg/ton of sodium oxide present insodium silicate (soluble glass) format, surrounding the iron oxideparticles—partly due to the effect of the carbonic acid gas generatedduring the reduction of hematite into magnetite carbonic acid gas,partly to the effect of the carbonic acid gas resulting from the addedcarbon dioxide—is converted to sodium carbonate. Thus, the sodium mixedinto the slurry (as opposed to soluble glass) does not interfere withthe digestion of iron oxides.

At this point remains 582 kg/t of residue slurry, of which about 200 kgis SiO₂ and 38 kg is TiO₂, which do not react with a sulfuric acid (orHCl) dissolution, and the remaining slurry contains 344 kg/ton of metaloxide, in which there are the rare-earth metal oxides as well. This isdissolved in 96% industrial strength sulfuric acid. The sulfuric acidneed of the reaction is assessed in such a manner, as if the entiremetal oxide mixture consisted of aluminum oxide Al₂O₃. Then theindustrial strength 96% sulfuric acid need of the 344 kg of metal oxidecomposition resulting from 1 ton of red mud is s 1033 kg. From the 344kg of metal oxide mixture 1201 kg of metal sulfate salt is formed, ofwhich 29 to 86 kg is rare-earth metal sulphates.

It can be seen that the rare-earth content of the red mud wasconcentrated to an average of sixty times (as compared to the startingconcentration), which is in a single metal-sulphate salt concentrate asrare-earth metal sulphate.

The rare-earth metals can be recovered from the sulphate salt byconventional procedures.

After the sulfuric acid dissolution process, the SiO₂ and TiO₂suspension is retained, with about 238 kg of dry matter content. Fromthis the valuable TiO₂ titanium dioxide is worth to extract, accordingto the P1200075 Patent Application, wherein due to the widely differingspecific gravities, the mixture is simply separated by centrifugation,or optionally by another suitable sedimentation method, the SiO₂ quartzsand and 38 kg/t TiO₂ titanium dioxide may be separated. The titaniumdioxide is commercialized. The 200 kg/t of quartz sand has been washedto high purity in the duration of the red mud processing. So it can becommercialized as high-grade foundry sand, but it is also suitable forcompressing ceramic and building blocks.

According to the process of the present invention the red mud pondsoccupying a lot of space, and being hazardous, can be eliminatedentirely and with great profits.

1. Process for recovering the iron oxide content of red mud, saidprocess comprising the step of converting the sodium oxide present inthe red mud in the form of soluble glass to sodium carbonate withcarbonic acid.
 2. Process for the utilization of red mud, said processcomprising the following steps: (a) providing red mud waste of aluminaproduction; (b) further, providing waste containing organic material,and converting of it to synthesis gas by high temperature pyrolysis; (c)converting the sodium oxide present in the red mud in the form ofsoluble glass to sodium carbonate with carbonic acid; (d1) magnetizingthe Fe₂O₃ hematite ferric oxide present in the red mud and separatingthe anti-ferromagnetic Fe₂O₃ hematite ferric oxide from the remainedslurry by magnetic separator; and/or (d2) converting the Fe₂O₃ hematiteferric oxide present in the red mud to Fe₃O₄ magnetite ferrous ferricoxide with synthesis gas, and separating the Fe₃O₄ magnetite ferrousferric oxide from the remained slurry by magnetic separator; and (e)treating the remained slurry obtained in steps (d1) or (d2) with strongacid, thus obtaining metal sulphate solution and SiO₂ and TiO₂suspension.
 3. The process as claimed in claim 2, said processcomprising the following steps: (d3) treating the Fe₂O₃ hematite ferricoxide obtained in step (d1) and/or the Fe₃O₄ magnetite ferrous ferricoxide obtained in step (d2) with the synthesis gas obtained in step (b),in which pure iron (Fe) is obtained.
 4. The process as claimed in claim3, said process comprising the following steps: (d4) treating the Feiron obtained in step (d3) with CO carbon monoxide gas, in which Fe(CO)₅iron pentacarbonyl is obtained.
 5. The process as claimed in claim 2,said process comprising the following steps: (e1) recovering the metalsand rare-earth metals from the metal sulphates and rare-earth sulphatesin the metal sulphate solution obtained in step (e) in a conventionalmanner.
 6. The process as claimed in claim 2, said process furthercomprising the following steps: (e2) separating the SiO₂ and TiO₂ slurryobtained in step (e) by conventional techniques, obtaining pure SiO₂quartz sand, and pure TiO₂ titanium dioxide.
 7. The process as claimedin claim 2, wherein 1:1 mole ratio CO+H₂ synthesis gas is used.
 8. Theprocess as claimed in claim 2, wherein the waste containing organicmaterial is a waste containing cellulose, especially selected from thegroup of waste paper, rice husking residues, straw, hemp, flax, theproducts of them, and the like; and the pyrolyisis is performed in adrum-type furnace.
 9. The process as claimed in claim 2, wherein thewaste containing organic material is a waste containing dioxin and/orfuran, especially selected from the group of waste oil, transformer oil,agricultural chemicals residue, and the like, or other industrialhazardous waste, especially selected from the group of bated leatherwaste, oil sludge, contaminated gas black, tarry waste, and thepyrolysis is performed in a plasma forge.
 10. The process as claimed inclaim 2, said process comprising the following steps: (a) providing redmud waste of alumina production (in the following: red mud); (b)further, providing waste containing organic material, and converting tosynthesis gas by high temperature pyrolysis; (c) converting the sodiumoxide present in the red mud in the form of soluble glass to sodiumcarbonate with carbonic acid; (d1) magnetizing the Fe₂O₃ hematite ferricoxide present in the red mud and separating the anti-ferromagnetic Fe₂O₃hematite ferric oxide from the remained slurry by magnetic separator;and/or (d2) converting the Fe₂O₃ hematite ferric oxide present in thered mud to Fe₃O₄ magnetite ferrous ferric oxide with synthesis gas, andseparating the Fe₃O₄ magnetite ferrous ferric oxide from the remainedslurry by magnetic separator; and/or (d3) treating the Fe₂O₃ hematiteferric oxide obtained in step (d1) and/or the Fe₃O₄ magnetite ferrousferri oxide obtained in step (d2) with the synthesis gas obtained instep (b), in which pure iron (Fe) is obtained; (d4) treating the iron(Fe) obtained in step (d3) with carbon monoxide gas (CO), in whichFe(CO)₅ iron pentacarbonyl is obtained, (e) treating the remained slurryobtained in steps (d1) or (d2) with strong acid, thus obtaining a metalsulphate solution and SiO₂ and TiO₂ suspension; (e1) recovering themetals and rare-earth metals from the metal sulphates and rare-earthsulphates in the metal sulphate solution obtained in step (e) in aconventional manner.
 11. The process as claimed in claim 10, saidprocess further comprising the following steps: (e2) separating the SiO₂and TiO₂ slurry obtained in step (e) by conventional techniques,obtaining pure SiO₂ quartz sand, and pure TiO₂ titanium dioxide.